1. Field of the Invention
Example embodiments relate to a calibrator for a filter and method for calibrating a filter. In particular, example embodiments relate to a calibrator for variably adjusting a time constant for an active Resistor-Capacitor (RC) filter and a method for calibrating an active RC filter.
2. Description of the Related Art
A signal processing system generally includes filters, which are used to reduce and/or remove unwanted components of a signal. Alternatively, filters may be used to enhance the desired components of the signal. In either case, filters play an important role in signal processing systems.
One characteristic of a filter is referred to as the cut-off frequency of the filter. For a high-pass or low-pass filter, the cut-off frequency of the filter corresponds to the frequency at which the output level of the filter decreases by about 3 dB. The cut-off frequency is generally represented by Equation (1) below, wherein f represents the cut-off frequency and τ represents the time constant of a RC circuit.
                    f        =                  1                      2            ⁢                                                  ⁢            π            ⁢                                                  ⁢            τ                                              (        1        )            Equation (2) is also helpful in relating the cut-off frequency to a RC circuit.τ=RC  (2)In Equation (2), R represents the resistance value in ohms of the filter and C represents the capacitance value in farads of the filter. Both the resistance value R and the capacitance value C are affected by external circumstances such as temperature, for example. As such, there are advantages to having filters, in which the cut-off frequency is variably adjusted to accommodate for external circumstances affecting the operation of a filter. Further, some signal processing systems require that a cut-off frequency of one or more filters included in the signal processing systems is variable.
As such, tunable filters have been designed that can be tuned to accommodate for external circumstances, such as temperature, as well as process variations that may result in a filter not operating according to a desired fixed time-constant. These tunable filters may be tuned based on a tuning code provided by a calibrator.
One example of a conventional calibrator for tuning a tunable filter is described in the article titled “Circuit Architectures for High Linearity Monolithic Continuous-Time Filtering” authored by A. M. Durham et al. and published in IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: Analog and Digital Signal Processing, vol. 39, no. 9, September 1992. A tuning code provided by conventional calibrators, such as the conventional calibrator described in the above article, is generally output based on a comparison of two analog signals. For example, the conventional calibrator described in the above article provides a tuning code once an output voltage of the calibrator corresponds to an analog ground voltage. However, these analog signals are generally directly or indirectly provided based on one or more reference voltages.
While conventional calibrators provide tuning codes intended to correct for the affects of external circumstances on various components, such as resistors, capacitors and operational amplifiers, these conventional calibrators generally do not consider the affects of the external circumstances and/or process variations on the reference voltages used by the calibrators to generate the tuning codes. If the one or more reference voltages used in conventional calibrators to provide a tuning code are affected by external circumstances, not accounted for by the calibrator, the tuning code provided by the conventional calibrators may not be as accurate as desired.